In late 2022, faced with a sharp increase in electricity prices and limited short-term infrastructure options, the University of Málaga collaborated with the Smart Decision Lab to promote energy-saving habits among staff and students. We designed a low-cost, behaviourally informed intervention centered on feedback, transparency, and social comparison. Energy efficiency ratings (from A++ to D), emoticons signalling performance, and intra-university rankings were displayed on posters and screens in university buildings. QR codes offered additional information and academic references. Complementary signage encouraged daily actions such as using natural light and turning off unused equipment. Using a staggered rollout, we observed that 85% of participating centers reduced energy consumption, with an average drop of over 9% (and 12% in centers with full signage). Centers without intervention showed only a 1.2% reduction, suggesting a clear causal link. This initiative highlights the power of informational nudges and social motivation to drive sustainable behaviour in institutional settings.